Internal combustion engines have been used to produce power and drive machines for over a century. From the beginning, internal combustion engines have undergone many improvements to become more efficient, more powerful, and/or less polluting. Various modifications to engine design along with many alternative fuel choices have been considered. In this disclosure, gaseous fuels are fuels that are in the gaseous phase at atmospheric pressure and temperature, and are gases that are combustible in an internal combustion engine of the disclosed type, with examples of such gaseous fuels being methane, ethane, propane, and other lighter flammable hydrocarbon derivatives as well as hydrogen and natural gas and other mixtures thereof. In particular, natural gas, being cleaner burning relative to conventional diesel fuels, and being abundant and more widely distributed around the world, has been receiving renewed attention as a substitute for more traditional fuels such as gasoline and diesel. That is, factors such as price, availability, energy security, and environmental concerns are leading more fuel users to consider alternative fuel options.
Natural gas has been used as a fuel in vehicular internal combustion engines for over fifty years. Historically, natural gas driven vehicles were naturally fumigated, meaning that natural gas was introduced into the intake manifold, with a mixture of fuel and intake air fed into the cylinders through the open intake valve. With such engines, the most common approach for igniting a gaseous-fuel/air mixture is to employ spark ignition because unlike liquid fuels like diesel, gaseous fuels are generally more difficult to ignite by compression ignition.
Generally there are two types of spark ignited gaseous-fuelled engines that have been commercialized, namely so-called lean burn engines that deliver an excess amount of oxygen to the combustion chamber, and engines that operate in a stoichiometric mode in which the gaseous-fuel/air mixture is controlled so that during combustion essentially all of the fuel is combined with essentially all of the free oxygen. That is, with an ideal stoichiometric fuel/air mixture there is just enough oxygen to burn essentially all of the available fuel. Lean burn engines and stoichiometric engines each have their advantages and disadvantages. For example, lean burn engines generally allow higher compression ratios and combined with lower throttling losses, this can provide higher efficiency and lower fuel consumption. A disadvantage of lean burn engines is that the presence of excess oxygen in the exhaust gas exiting the combustion chamber makes a lean burn engine incompatible with modern three way catalyst aftertreatment subsystems, which means that a more expensive aftertreatment subsystem is required to reduce NOx levels.
Stoichiometric engines normally have lower compression ratios compared to lean burn engines, which normally results in lower efficiency and/or lower power output, but the combustion products are compatible with modern three-way catalyst aftertreatment subsystems so this has helped stoichiometric engines to meet recent emissions standards without requiring the more complex and more expensive aftertreatment subsystems needed by lean burn engines. For example, the applicant's related company, Cummins Westport Inc. recently offered an advanced natural gas engine that operates in a stoichiometric mode, with exhaust gas recirculation and spark ignition, and engines with this combination of features are referred to herein as SESI engines. Compared to earlier engines, it uses relatively high rates of cooled exhaust gas recirculation (EGR) to reduce excess air and thereby reduce the production of NOx during combustion, while also lessening the likelihood of combustion knock.
Another approach for natural gas engines is not stoichiometric and involves the use of compression ignition to ignite the fuel/air mixture (the diesel principle) instead of spark ignition. Higher compression ratios are used than those used in spark ignited engines, thus allowing for greater power and efficiency. However, as noted previously, a charge consisting of gaseous fuel and air is difficult to ignite by compression alone without the use of an ignition assisting device, such as the ignition of a more readily ignited pilot fuel, such as a small amount of diesel fuel, or a glow plug or other hot surface.
When a pilot fuel is used it is typically directly injected into the combustion chamber of the engine cylinders to initiate ignition of the primary gaseous fuel. The pilot fuel mixes with air in the combustion chamber, ignites as a result of the pressure/temperature conditions therein, and in turn ignites the gaseous fuel. The amount of pilot fuel required can be very small, for instance approximately 1% of the total fuel present. Such pilot operation is sometimes referred to as “micropilot” and this term is defined herein to mean this.
Engines using a compression ignition approach and operating primarily with fumigated gaseous fuel are often referred to as “dual fuel” engines and are referred to herein as such. Dual fuel engines can inject diesel pilot fuel directly into the combustion chamber for ignition purposes and EGR can be employed. However, this approach uses an excess amount of air since it does not employ a throttle and therefore it is not stoichiometric, and like lean burn engines, dual fuel engines require more complicated and expensive exhaust treatment to treat emissions. An advantage of dual fuel engines is that they allow for a relatively easy retrofit of existing diesel engines. In addition, it allows for the use of diesel only (100% pilot fuel) should that prove desirable or necessary.
In general, engines can be made more efficient, more powerful, and less polluting with more precise control over the timing for fuel injection, the quantity of fuel injected, and the rate of fuel injection during an injection event. Better efficiency and emissions can be achieved in a gaseous-fuelled engine if the gaseous fuel is injected directly into the cylinders under high pressure with the timing for start of injection beginning near the end of the compression stroke of the piston. This approach reduces the potential for combustion knock and allows gaseous-fuelled engines to be operated with the same compression ratios as conventional diesel engines. However, this requires a more complicated and expensive fuel supply subsystem which can deliver both the primary gaseous fuel and the pilot fuel at injection pressures of at least 200 bar.
Advanced engines using direct injection of gaseous fuels into the combustion chambers of the engine cylinders at such injection pressures are disclosed, for example, in co-owned U.S. Pat. Nos. 6,073,862, 6,439,192 and 6,761,325. Therein and herein, these engines are referred to as high pressure direct injection engines or “HPDI engines”. While offering advantages compared to other gaseous-fuelled engines in terms of power, efficiency and high potential substitution percentages of primary gaseous fuel for diesel, such engines operate in a lean mode, with excess air (not stoichiometric), like conventional diesel engines. Accordingly, to comply with current emissions requirements in many jurisdictions, compared to stoichiometric engines, HPDI engines typically require a more complicated and expensive aftertreatment subsystem for treatment of the exhaust.
A variation of HPDI uses a glow plug or other hot surface ignition device instead of a pilot fuel, to ignite the gaseous fuel. Engines that use this approach are disclosed, for example, in co-owned U.S. Pat. Nos. 6,845,746, 7,077,115 and 7,281,514. In the disclosed preferred embodiments, a gaseous fuel is injected directly into the combustion chamber, with the timing for start of injection being late in the compression cycle near or at top dead center and at about the same injection pressure as HPDI engines that employ a pilot fuel.
Numerous other engine embodiments have been contemplated and disclosed in the art where the primary fuel is other than natural gas. For instance, the Southwest Research Institute (SWRI), in U.S. Pat. No. 6,679,224, discloses a diesel engine employing EGR that is adapted to work temporarily under stoichiometric conditions, and in particular to provide a means for regenerating a lean NOx trap without introducing unburned fuel into the exhaust stream of the engine, or requiring additional substances for operating the engine or after-treatment device. The primary fuel is diesel, and it teaches using a second fuel such as distilled diesel, gasoline, natural gas, liquid petroleum gas (LPG), or hydrogen, which is temporarily injected into the intake manifold to premix with air before it is introduced into the combustion chamber. In U.S. Pat. No. 7,389,752, SWRI also teach an engine embodiment where gasoline is the preferred primary fuel and lubricating oil is the micro pilot ignition fuel. A high level of EGR (for instance 25-60%) can be used. Neither of these disclosures by SWRI teaches using a gaseous fuel as the primary fuel, and adjusting the method of operating the engine in a different way from a conventional liquid-fuelled engine to take advantage of the different properties of gaseous fuels such as, for example, the combustion of such gaseous fuels producing less particulate matter, also known as soot, which can allow higher levels of EGR without the effect of recirculating large amounts of soot, and the generally higher flammability limits and longer ignition delays that can help to reduce the danger of combustion knocking.
Even though internal combustion engines have undergone continuous improvement for more than a century, the combustion process in an internal combustion engine is complex and even now it is not fully understood. There are many variables and combinations of features that have not been tried and without investigation by computer modeling and/or experimental testing, the effect of a previously untried combination can not be accurately predicted. As discussed above, with respect to gaseous-fuelled engines, there have been approaches that have used spark ignition, pilot ignition, hot surface ignition, and there have been lean burn engines, stoichiometric engines, and there have been port injected fumigated engines with pre-mixed fuel-air mixtures and directly injected stratified fuel-air mixtures, and there have been engines that use three way catalysts and engines that use relatively more complex aftertreatment subsystems such as selective catalytic reduction, which requires the addition of a reductant such as urea.
A concern with engine technology in general is the need to prevent unacceptable combustion knock which can become more problematic as in-cylinder temperatures get higher and/or with higher compression ratios and/or lower octane fuels, and so on. Various techniques have been suggested in the art to control or reduce combustion knock. For instance, U.S. Pat. No. 7,028,644 discloses adding hydrogen to avoid combustion knocking and to allow for higher levels of cooled EGR in spark ignited, gasoline engines with high compression ratios. U.S. Pat. Nos. 7,290,522 and 7,461,628 disclose two mode engines with addition of hydrogen or varied amounts of injected ethanol to respectively prevent combustion knock.
Much work has been done to improve engine performance and provide for alternative fuel use. Among known gaseous-fuelled engine technologies, HPDI has been shown to yield the highest performance and efficiencies, which makes HPDI the preferred choice for certain applications. However, for less demanding applications, which do not require such high performance, there is a need for an engine that is simpler and less expensive. The present technique addresses this and other needs.